1. Field of the Invention
The present invention relates to a compact zoom lens system having a high zoom ratio and including a range of wide angle of view, in which there is no need of securing a sufficient back focal length, such as a compact zoom lens system used for 35 mm lens-shutter type camera, and to a focusing operation thereof.
2. Description of the Relating Art
Conventionally, as a compact zoom lens system in which there is no need of securing a sufficient back focal length, there have been known various types of the zoom lens system, for example two, three and four component type zoom lens systems. However, in almost all types of such a zoom lens system the zoom ratio is less than 3 so that it is desirable that the zoom lens system with a high zoom ratio more than 3 is provided for the lens-shutter type camera.
On the other hand, a lot of the zoom lens systems whose zoom ratio is more than 3 have been provided as for a single lens reflex camera. However, in this type of the zoom lens system having a range of a wide angle of view, the total length of the whole lens system is very long because of the limitation of the back focal length. Therefore, above-mentioned zoom lens system is not suitable for the lens-shutter type camera.
Further, in most of the zoom lens system the front lens unit located at the object side end of the lens system is utilized as a focusing lens unit for avoiding the complication of the structure, so that the shifting amount of the front lens unit is a constant in any focal length condition for focusing on the same object. However, if the front lens unit functions as a focusing lens unit in the zoom lens system including a range of a wide angle of view and having the front lens unit of positive refractive power, a diameter of the front lens unit must be large for securing the accurate illuminance at the marginal point of the film in the closer object focusing condition.
Furthermore, a limitation for a shifting amount of the focusing lens unit in any focal length is considerably reduced in the recent auto focus camera which drive the focusing lens unit by means of the driving system. But it is desirable that the weight of the focusing lens unit is light and a shifting amount thereof is small for compactness of the driving system.